The process of dewatering waste sludges begins with a conditioning or flocculation step. Flocculation is the mechanism wherein suspended matter in the waste stream, which is present as a disperse phase is charge neutralized and thereby coagulated or coalesced. Subsequent treatment of this flocculated mass involves dewatering which is carried out mechanically.
Waste matter which is produced biologically, i.e., a secondary sludge, can originate, from an industrial process or from municipal sewage. Such a sludge consists of high levels of cell bodies, most often as finely divided organic sediments, and requires a conditioning before dewatering. Also, the solids are low, which makes the conditioning all the more difficult. Blending with other sludges may be carried out to increase these solids.
The conditioning of the waste matter may be effected by the addition of an inorganic filler such as clay or fly ash. The purpose of the filler is to provide greater surface area, weight and density to the mass. Subsequently, a cationic polymer is often added to accomplish charge neutralization since both the cellular bodies and the clay bear negative surface charges. A flocculation results and the mass is then more readily dewatered by mechanical means. A popular and effective device for the latter dewatering stage is one that provides a pressing action to facilitate release of free water. Belt-filter, filter and screw presses are examples of these devices.
Reference is made to the publications of Poduska and Stroupe and Poduska and Collins, Proc. Ind. Waste Conference 198135437 and Environmental Technology Letters 1547 (1980), respectively, in which dewatering of activated sludges derived from chemical, plastics and fiber production process wastes are described.
In these reports the advantages of the belt filter press as a mechanical aid to dewatering along with clay and cationic polymers are elucidated. Process optimization is demonstrated using a laboratory apparatus for studying the effects of polymer dose, dilution, mixing intensity and duration, gravity and compression drainage, sludge pH and temperature on flocculation.
The type of polymer used with clay in the aforementioned publications is a copolymer of acrylamide and a quaternary ester such as methacroyloxyethyl trimethylammonium chloride or methosulfate. Usually a fairly high mole % of the cationic component is incorporated, e.g., 20 to 60 mole %, but an even higher ratio may be used and sometimes the homopolymer of the cationic monomer is found effective.
The molecular weight of the polymer applied along with the clay is above one million and may exceed 5 to 10 million. Usually the higher molecular weight polymers are preferred because they are more effective. The clay employed is preferably a bentonite but other clays such as Kaolin clay may also be used.